1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, relates to a semiconductor device for converting an analog signal into digital data and outputting the digital data.
2. Description of the Related Art
In recent years, regulation for the safety of vehicles has been enhanced in succession in Japan and the United States. According to the TREAD Act (Transportation Recall Enhancement, Accountability and Document Act) enforced in North America, new automobiles sold in and after the year 2006 are obligated to be equipped with an automobile tire-pressure monitoring system. For this reason, a technique for mounting a sensor in a tire is presently developed to measure a pneumatic pressure and a temperature. The technique is a system for mounting a sensor unit in a valve section of the tire and monitoring all four tires individually. Therefore, it is possible to carry out monitoring with a high accuracy, and there is a merit that the monitoring of the tire pressure can be carried out even when an automobile is in a stop.
The sensor unit mounted to the tire is provided with a pressure sensor for outputting an analog voltage signal in response to the tire pressure and a semiconductor device for converting the analog voltage signal outputted from the pressure sensor into a digital signal and outputting the digital signal. In general, such a semiconductor device has an AD conversion circuit to convert an analog signal into a digital signal. Usually, a predetermined voltage is supplied from a power supply for the sensor to the pressure sensor. The pressure sensor generates the analog voltage signal by use of the supplied voltage, and supplies the generated analog voltage signal to the semiconductor device. The AD conversion circuit is connected to a power supply for supplying a reference voltage, and a constant voltage is supplied from the reference voltage supply. The AD conversion circuit converts the analog signal outputted from the pressure sensor into the digital signal by use of the reference voltage supplied from the reference voltage supply, as described in Japanese Laid Open Patent Application (JP-A-Heisei, 6-243549) as a conventional example.
When the pneumatic pressure of the tire mounted to a vehicle is measured, the sensor unit is generally provided for the valve section of the tire and so on, to measure the tire pressure. The temperature of the tire is different in case that the vehicle is running and in case that the vehicle is in a stop. That is, the temperature of the tire of the vehicle on running is increased due to friction with a road surface, compared with the vehicle in a stop. If the temperature of the tire is increased, the temperature of the sensor unit mounted to the valve section of the tire is increased, too.
FIG. 1 is a block diagram showing a configuration of a tension measuring circuit as a semiconductor device described in the above conventional example. The tension measuring circuit is configured from a sensor unit 110, a control unit 120, an amplifier unit 106, and an AD conversion 108. As shown in FIG. 1, a direct-current power supply 134 is connected to the sensor unit 110.
If the temperature changes, a voltage outputted from a battery varies. When a battery connected to the pressure sensor and a battery connected to the AD conversion circuit have different temperatures and discharging characteristics, the voltage of the battery connected to the pressure sensor varies. For this reason, the reference voltage supplied to the AD conversion circuit varies. Therefore, an output value from the AD conversion circuit deviates from a correct value since each variation characteristic is different. As a result, the pressure applied to the pressure sensor cannot be correctly outputted. In order prevent variation of the power supply voltages for the sensor and the reference voltage, a technique for providing a power supply circuit of high accuracy is known. That is, a temperature change in the sensor unit can be dealt with by providing the power supply circuit in which an output voltage does not easily shifted for the temperature change.
However, such a power supply circuit of high accuracy is generally expensive. Therefore, provision of the power supply circuit of high accuracy leads to the increase in manufacturing costs of the semiconductor device. In addition, the power supply circuit of high accuracy generally has a large circuit area. As a result, the circuit area of the semiconductor device provided with such a power supply also increases.
A semiconductor device capable of outputting appropriate data in correspondence to the voltage variation is demanded even when the reference voltage of the AD conversion circuit shifts due to the effect of the temperature change around the circuit and so on. Further, a technique is desired, for forming a semiconductor device that is difficult to receive the effect of the voltage variation, without causing the increase in the manufacturing costs and the circuit area.